The long-term objectives of this proposal are to understand how specific genetic changes cause abnormal fate determination and growth regulation within adult epithelial stem cells and how these changes promote tumor formation from stem cells. The LEF/TCF family of transcription factors play an essential role in stem cell-mediated skin homeostasis. We have shown that human sebaceous tumors are similar to mouse tumors induced by a dominant negative Lef-1 gene and we recently identified mutations within the LEF-1 gene of sebaceous skin tumors. In addition, TCF-4 mutations occur in a significant proportion of colon cancers. Thus, our HYPOTHESIS is that LEF/TCF mutations cause abnormal stem cell fate determination and growth that promotes the formation of sebaceous tumors of the skin. Specific Aims: (1) Determine the cellular mechanisms by which dysregulated LEF-1 promotes tumorigenesis in multi-potent stem cells of human skin. (2) Determine the cellular and molecular properties of a new sebaceous tumor cell line showing deficient LEF-1 signaling. (3) Survey the spectrum and incidence of LEF/TCF mutations in human sebaceous skin tumors and correlate with tumor morphology. In order to accomplish these aims, we will use a novel system to isolate adult epithelial stem cells of human skin and study the cellular changes induced by altered LEF-1 signaling. We will use in vitro assays, as well as molecular and biochemical tests to characterize a new sebaceous tumor cell line. Lastly, a large archival tissue bank of sebaceous tumors will be screened by several molecular techniques to identify mutations within the LEF-1, TCF-3 and TCF-4 genes. Relevance: Patients with Muir-Torre syndrome, a genetically inherited defect of DNA repair, develop many sebaceous skin tumors and are at a high risk of colon cancer. Sebaceous tumors of the eyelid require removal of the eye in almost 25% of patients. This work should provide significant insights into the pathogenesis of sebaceous skin tumors, may lead to new ways of preventing or treating these tumors and may have broader implications for other cancers arising from adult epithelial stem cells.